There is a posting site that allows anybody to view a posted video image at any time. There is a possibility that a video image is illegally posted. For example, posting of a copy of a TV program, a movie, or the like infringes a copyright. In recent years, a huge number of video images have been posted on sites. In such a situation, it is very difficult to determine whether posted video images are legal or illegal by visually checking the video images or performing a search using keywords. Thus, there is a desire for a technique that allows it to efficiently evaluate identity of video images or similarity between video images.
A technique to determine whether video images are identical is disclosed, for example, in Japanese Laid-open Patent Publication No. 2011-118498. In this technique, relative luminance is calculated for various sub-areas of a video image, and a result is represented by one of three values, −1, 0, and 1 as a feature value for each video image. The calculated feature values are compared between video images to determine whether the video images are identical. More specifically, when the comparison indicates similarity higher than a threshold value, the video images are determined to be identical. In a technique disclosed, for example, in Japanese Laid-open Patent Publication No. 2009-3889 or Japanese Laid-open Patent Publication No. 2001-109494, the comparison is performed using a phase of a frequency.